1. Field of the Invention
The invention relates to an apparatus and method for determining degradation of a catalyst provided in an exhaust passage of an internal combustion engine.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-6-159048, for example, discloses an apparatus which calculates an oxygen storage capacity (OSC) of a catalyst provided in an exhaust passage of an internal combustion engine and then determines degradation of the catalyst based on that calculated oxygen storage capacity. This apparatus calculates the oxygen storage capacity of the catalyst by calculating the amount of excess oxygen or the amount of deficient oxygen in the exhaust gas flowing into the catalyst from the time that a control target value of the air-fuel ratio (hereinafter referred to as “target air-fuel ratio”) is changed from rich (or lean) to lean (or rich) until the time that the output of an oxygen sensor downstream of the catalyst reverses.
When degradation of the catalyst progresses, however, the output of the oxygen sensor may reverse before the oxygen absorbing/releasing reaction progresses sufficiently in the mid to rear portion of the catalyst. FIG. 14 is a view showing the change in temperature of that accompanies an oxygen absorbing reaction in the catalyst. FIG. 14A is a view showing settings of the target air-fuel ratio. FIG. 14B is a view showing the change in the oxygen sensor output. FIG. 14C shows the change in temperature at the front portion of the catalyst, FIG. 14D shows the change in temperature at the middle of the catalyst, and FIG. 14E shows the change in temperature at the rear portion of the catalyst. As shown in FIG. 14A, when the target air-fuel ratio is changed from a value of 13.8 on the rich side to a value of 15.2 on the lean side, the oxygen absorbing reaction progresses in the catalyst. The oxygen absorbing reaction in the catalyst is an exothermal reaction. Therefore, as the oxygen absorbing reaction progresses from the front portion to the rear portion of the catalyst, a temperature rise peak (hereinafter simply referred to as “peak”) P1 occurs first at the front portion of the catalyst, as shown in FIG. 14C. Next, a peak P2 occurs at the middle of the catalyst, as shown in FIG. 14D, after which a peak P3 then occurs at the rear portion of the catalyst, as shown in FIG. 14D. Thus it is possible to know how far the oxygen absorbing reaction has progressed in the catalyst based on these peaks P1 to P3. As shown in FIG. 14E, the oxygen sensor output reverses (at time tr in the drawing) before peak P3 occurs at the rear portion of the catalyst so the oxygen storage capacity of the entire catalyst is unable to be calculated. As a result, if degradation of the catalyst is determined based on the oxygen storage capacity of the catalyst, it is not able to be made accurately. In addition, a catalyst which has degraded to a large degree is unable to be accurately detected.